Process for preparing solutions of anionic organic compounds

ABSTRACT

Described is a process for preparing concentrated solutions or suspensions of anionic organic compounds which comprises a) acidifying an aqueous solution or suspension of an anionic organic compound containing salts and/or impurities to a pH of 4.5 or less, if the pH is above this value, so that b) the anionic organic compound is water-insoluble and precipitates in the form of the free acid, c) bringing the suspension to a salt content below 2% by weight, based on the total weight of the retentate, by means of ultrafiltration with a membrane having pore diameters of from 0.001 to 0.02 μm, and d) optionally washing out the salts with water having a pH below 4,5, e) optionally carrying out acid-free washing with water thereafter, and then f) concentrating, so that the amount of anionic organic compound is 5-50% by weight, and g) if desired, bringing the anionic organic compound into solution by adding an appropriate base.

The present invention relates to a process for preparing solutions ofanionic organic compounds, to the solutions thus prepared and to the useof such solutions. By anionic organic compounds are meant, inparticular, dyes and optical brighteners and also intermediates for thepreparation thereof.

In recent years, the use of concentrated aqueous solutions, of dyes andoptical brighteners, for example, has increased in importance, owing tothe advantages which such solutions possess over the correspondingpowder forms. Through the use of solutions, the difficulties associatedwith the formation of dust are avoided and the users are freed from thetime-consuming and often difficult dissolving of the powder in water.The use of concentrated solutions has also been promoted by thedevelopment of continuous processes for dyeing or optically brighteningpaper, since with these processes it is judicious to introduce thesolution directly into the hollander or to add it at any other suitablepoint in the papermaking process.

With certain dyes and optical brighteners, however, it is difficult toformulate concentrated solutions, since such solutions, especially ifthey contain significant amounts of inorganic salts, have a tendency togel. In that case it is virtually impossible to remove salts from suchgels and/or to purify them by filtration and washing.

Furthermore, when the concentrated solutions are stored, especially attemperatures below room temperature, a relatively frequent occurrence isthe formation of deposits which can be brought back into solution onlywith considerable effort, if at all. Furthermore, concentrated solutionsof anionic dyes or optical brighteners, which are appropriate as thecommercial form, ought to give clear solutions when diluted for thepreparation of the dyebaths, these solutions containing from about 1 to3% of dye or optical brightener, without precipitation, and should do sowithin a very wide pH range as well.

It was an object of the present invention to provide suitableconcentrated solutions of such dyes and optical brighteners, and alsointermediates for the preparation thereof, where the abovementioneddifficulties do not occur.

It has now been found that, by means of the process described below, itis possible simply and cost-effectively to prepare concentratedsolutions which satisfy the abovementioned requirements in anoutstanding way. The process constitutes a simple and cost-effectivemethod of converting anionic organic compounds which are in a poorlysoluble salt form into a readily soluble form by intermediate conversionof some or all of the acidic groups into the acid form and subsequentlycarrying out neutralization with appropriate bases.

DE-A-199 27 398 has already described a process for preparingformulations comprising dyes and/or brighteners, in which an aqueoussuspension comprising dyes and/or brighteners is desalinated using amicrofiltration membrane having pore diameters of from 0.05 to 40 μm. Ithas now been found that, surprisingly, the desalination is carried outadvantageously by ultrafiltration with a membrane having pore diametersof from 0.001 to 0.02 μm. When using such a membrane having markedlysmaller pore sizes, it is found contrary to expectation that backwashingis necessary less frequently, thereby raising the capacity of thedesalination unit; the performance of the membranes is stable at a highlevel. In particular, fewer problems occur if the dyes and/orbrighteners are present in non-uniform crystal sizes or are reduced insize by the process (very high circulation).

The present invention accordingly provides a process for preparingconcentrated solutions or suspensions of anionic organic compounds,which comprises

-   -   a) acidifying an aqueous solution or suspension of an anionic        organic compound containing salts and/or impurities to a pH of        4.5 or less, if the pH is above this value, so that    -   b) the anionic organic compound is water-insoluble and        precipitates in the form of the free acid,    -   c) bringing the suspension to a salt content below 2% by weight,        based on the total weight of the retentate, by means of        ultrafiltration with a membrane having pore diameters of from        0.001 to 0.02 μm, and    -   d) optionally washing out the salts with water having a pH below        4.5,    -   e) optionally carrying out acid-free washing with water        thereafter, and then    -   f) concentrating, so that the amount of anionic organic compound        is 5-50% by weight, and    -   g) if desired, bringing the anionic organic compound into        solution by adding an appropriate base.

By anionic organic compounds are meant, in particular, dyes and opticalbrighteners and also intermediates for the preparation thereof.

Suitable dyes for the process of the invention include anionic dyeswhich are stable and insoluble in water at pH levels below 4.5. Thesedyes may belong to any class. They comprise, for example, dyescontaining at least one sulfonic acid and/or carboxylic acid group, fromthe following dye classes: metal-free or metallic monoazo, disazo andpolyazo dyes, pyrazolone, thioxanthone, oxazine, stilbene, formazan,anthraquinone, nitro, methine, triphenylmethane, xanthone,naphthazarine, styryl, azastyryl, naphthoperinone, quinophthalone, andphthalocyanine dyes. These dyes may contain one or more fibre-reactivegroups in the molecule.

With preference, the dyes involved are azo dyes containing at least onesulfo group, and among these are preferably those known as azo directdyes, for example those listed in the Colour Index, Third Edition,Volume 2 (The Society of Dyers and Colourists, 1971). A furtherpreferred class is that known as the stilbene dyes.

Particular preference is given to the dyes suitable for dyeing paperand, of these, more particularly the dyes of the formula

in which KK denotes the radical of a coupling component.

Preferably, KK is a coupling component of the formula

in which

-   -   Y₁ and Y₂ independently of one another are ═O, ═NH, or        ═N—C₁-C₄alkyl,    -   Y₃ is ═O, ═S, ═NR or ═N—CN, where R is hydrogen or C₁-C₄alkyl,        and    -   R₁ and R₂ independently of one another are each hydrogen,        unsubstituted or substituted alkyl or unsubstituted or        substituted phenyl.

In the above formula (2), only one tautomeric form is indicated with thecoupling component; however, this formula is intended to embrace theother tautomeric forms as well.

Unsubstituted or substituted alkyl groups R₁ and/or R₂ are for examplemethyl, ethyl, n- or isopropyl, n-, sec- or tert-butyl, straight-chainor branched pentyl or hexyl or cyclohexyl, it being possible for theseradicals to be substituted one or more times by, for example, OH,C₁-C₄alkoxy or C₁-C₄hydroxyalkoxy.

Examples of suitable substituted alkyl radicals are methoxymethyl,ethoxymethyl, ethoxyethyl, ethoxypropyl, n-propoxymethyl, butoxyethyland 2-hydroxyethoxypentyl.

Unsubstituted or substituted phenyl R₁ or R₂ may be substituted one ormore times, for example by C₁-C₄alkyl, C₁-C₄alkoxy, halogen, such asfluoro, chloro or bromo, or nitro.

R₁ and R₂ are preferably hydrogen or C₁-C₄alkyl.

Y₁ and Y₂ are preferably ═O or ═NH, it being additionally preferred forY₁ and Y₂ to be identical.

Y₃ is preferably ═O, ═S, ═NH or ═N—CN and in particular is ═NH.

The dyes of the formula (1) are known or may be prepared conventionally.

The stilbene dyes comprise complex dye mixtures resulting from thecondensation of 4-nitrotoluene-2-sulfonic acid with itself or with otheraromatic compounds. Their structure is defined by the manner of theirpreparation. Examples of suitable stilbene dyes are those described inthe Colour Index, Third Edition, Volume 4 (The Society of Dyers andColourists, 1971) under the constitution numbers 40,000 to 40,510.

Suitable dyes in the process of the invention include preferably thedyes Direct Yellow 11 and its derivatives Direct Yellow 6 and DirectOrange 15, obtainable by reductive steps incorporated additionally intothe synthesis.

Suitable optical brighteners for the process of the invention includebrighteners of various classes which contain sulfo groups and/orcarboxyl groups, examples being bistriazinylaminostilbenes,bistriazolylstilbenes, bisstyrylbiphenyls or bisbenzofuranylbiphenyls,bisbenzoxalyl derivatives, bisbenzimidazolyl derivatives, coumarinderivatives or pyrazoline derivatives.

For example, the process of the invention is suitable for preparingconcentrated solutions of the following optical brighteners:

Suitable intermediates for the process of the invention include, inparticular, anionic intermediates used for synthesizing dyes or opticalbrighteners.

Such intermediates include in particular aromatic sulfonic acids whichadditionally carry one or more further substituents, examples beingamino, nitro, alkyl or hydroxyl.

Particularly suitable intermediates are, for example, the following:2-amino-5-hydroxynaphthalene-7-sulfonic acid, 4-aminotoluene-2-sulfonicacid, dehydroparathiotoluidinesulfonic acid,4,4′-diaminostilbene-2,2′-disulfonic acid,4,4′-dinitrostilbene-2,2′-disulfonic acid,4,4′-diaminodiphenylamine-2-sulfonic acid and 4-nitrotoluene-2-sulfonicacid.

The process of the invention is carried out in detail as follows:

The normal starting point is an aqueous synthesis solution or synthesissuspension which as well as the anionic organic compound containsgreater or lesser amounts of starting materials, byproducts, salts orother impurities. Where, in contrast, the anionic organic compound is insolid form or in the form of a slurry or paste, it is first suspended inwater to give an aqueous suspension or solution.

If the anionic organic compound is already in the form of the free acidtherein, then the ultrafiltration is carried out immediately. If, on theother hand, it is in salt form, then the first stage of the process ofthe invention comprises converting the salt into the free acid.

In the case of compounds having two or more sulfo groups, it issometimes advantageous to perform the conversion into the free acid intwo or more stages at different pH values and/or temperatures, or toconvert only some of the sulfo groups into the free acid.

To prepare the free acid, an aqueous solution or suspension of theanionic organic compound containing salts and/or other impurities isacidified to a pH of 4.5 or below and is stirred or mixed until theanionic organic compound has undergone almost complete conversion intothe free acid and is therefore water-insoluble, and precipitates. Thistakes place preferably by the addition of a strong inorganic acid, forexample hydrochloric acid or sulfuric acid, until the desired pH hasbeen reached. The conversion takes place advantageously at a temperaturebetween 15 and 140° C., in particular between 20 and 95° C.

The optimum pH, the temperature, the concentration and the duration ofmixing must be adjusted for the anionic organic compound and for thedesired degree of substitution. The optimum conditions are easy todetermine by means of corresponding experiments.

In the case of anionic organic compounds which are difficult to convert,it may be useful first to subject the solution or suspension to partialdesalination and only then to carry out the conversion into the freeacid. This may be done, for example, by nanofiltration or intermediateisolation of the anionic organic compound. Furthermore, it is possibleto use special synthesis techniques to generate low-salt synthesissolutions, such as simultaneous diazotization and coupling, for example.Furthermore, an anionic organic compound which has been only partlyconverted into the free acid can be washed until low in salt and thenfurther acid can be added and can be stirred in or mixed in, at anelevated temperature if desired.

Washing and conversion into the free acid may also be carried outcontinuously in succession by circulating the suspension through anultrafiltration module which is connected in series with a reactor forconversion into the free acid and, if desired, for heating.

Ultrafiltration takes place by means of the methods which are customaryper se and which are common knowledge, using the known membranes. Thesemembranes may comprise acid-resistant organic or inorganic material.Particularly suitable membranes are ceramic membranes, forultrafiltration particularly those having a pore size of from 0.005 to0.01 μm.

The temperature during the ultrafiltration is approximately between roomtemperature and about 95° C., preferably between 50 and 85° C. Thepressure depends, inter alia, on the nature of the membrane, but isusually between 2 and 10 bar, preferably between 4 and 8 bar.

Washing and concentration by means of ultrafiltration is carried outuntil the desired salt content and the desired concentration of anionicorganic compound have been achieved. Normally, the aim is for aninorganic salt content of below 2% by weight, preferably below 0.5% byweight, based on the total weight of the suspension.

Following ultrafiltration, the amount of anionic organic compound ispreferably between 5 and 50% by weight, in particular between 10 and 40%by weight, based on the total weight of the suspension.

Following ultrafiltration, the low-salt or salt-free suspension may beadmixed with any desired bases in order to give readily soluble salts ofthe anionic organic compounds with any desired cations. Examples ofsuitable bases are LiOH, NH₄OH, or organic amines, such as aC₄-C₁₂trialkylamine, C₄-C₁₂diamine, C₂-C₁₅alkanolamine orpolyglycolamine, for example. It is preferred to use LiOH, NH₄OH or analkanolamine.

The resultant dye solutions or brightener solutions may be used directlyin this form or, if desired, after dilution. Alternatively, they can bedried in a customary manner and used as powders or granules.

In the examples below the parts and percentages are by weight unlessstated otherwise. The temperatures indicated are in degrees Celsius.

EXAMPLE 1

96 parts of dehydrothio-p-toluidinesulfonic acid are suspended in 600parts of water at 60° and are dissolved at a pH of from 7.5 to 8 byadding 25 parts of 50% sodium hydroxide solution. Following completedissolution, 46.3 parts of sodium nitrite solution (46 parts in 100parts of water) are added. The resulting solution is metered over thecourse of 20 minutes into 90 parts of 32% hydrochloric acid and a littleice, the temperature being maintained between 15 and 20° by continualaddition of ice. Stirring is continued for 30 minutes, to give about1400 parts of a yellow suspension. Prior to coupling, any excess sodiumnitrite is removed using sulfamic acid.

40.5 parts of barbituric acid are added to the resulting suspension, andthe mixture is stirred for 15 minutes. Then about 46 parts of 50% sodiumhydroxide solution are metered in over the course of 3 hours so as tomaintain a pH of 3.3. When sodium hydroxide solution is no longer takenup, the mixture is heated to 75° and, at this temperature, 69 parts of32% hydrochloric acid are added over 5 minutes, after which the mixtureis stirred at 80 to 85° for 2 hours. During this time, theorange-coloured suspension, containing the sodium salt of the dye, isconverted into the yellow suspension of the free acid of the formula

The volume is approximately 1800 parts.

The suspension is cooled down to 50 to 60° and the volume is reduced bya third by ultrafiltration in a customary ultrafiltration unit fittedwith membrane candles (ceramic membrane on Al₂O₃ support material, poresize 0.005 to 0.02 μm).

Then washing is carried out in the same unit first with 3600 parts ofdeionized water adjusted to a pH of 1.0 with HCl and then with 2400parts of deionized water adjusted to a pH of 4.5 with HCl. Thereafter,the mixture is concentrated to 900 parts by volume.

A solution of 6.5 parts of lithium hydroxide 1H₂O and 34 parts oftriethanolamine in 80 parts of water is added to the resultingsuspension. This gives a dark, clear solution having a pH of about 7.Addition of 80 parts of water gives 1100 parts of a storage-stable dyeformulation having a sodium content of less than 300 ppm and a dyecontent of 11.6% (calculated as free acid).

If the procedure is carried out without ultrafiltration and the dye isisolated from the suspension in the free acid by filtration and washingof the presscake with water, then in the course of normal operatingpractice using conventional filter presses it is impossible to obtainthe desired low sodium content.

EXAMPLE 2

The procedure described in example 1 is repeated but using, instead ofbarbituric acid, an equivalent amount of cyaniminobarbituric acid andconducting the conversion into the free acid at 85° using 10% HCl.Concentration is then carried out by a factor of 2 in the sameultrafiltration unit. This is followed by washing with 4 times thevolume of deionized water which has been adjusted to a pH of 3.0 usingHCl.

Following further operation as in example 1, with just triethanolamineas base, a storage-stable formulation of the dye of the formula

is obtained which has a chloride content of less than 0.1% and a sodiumcontent of less than 0.05%.

EXAMPLE 3

The procedure described in example 1 is repeated but using, instead ofbarbituric acid, an equivalent amount of 2,4,6-triaminopyrimidine andcarrying out the conversion into the free acid at 60° and a pH of from 1to 2. Concentration is then carried out in the same ultrafiltration unitby a factor of 2. This is followed by washing with 5 times the volume ofdeionized water which has been adjusted to a pH of 1.0 using HCl.

Following further operation as in example 1, a storage-stableformulation of the dye of the formula

is obtained, using as the base mixture an equivalent amount of a 1:1mixture of 3-diethylamino-1-propylamine and diethanolamine.

EXAMPLE 4

800 parts of water were charged to a flask with plane-ground joints, and120 parts of NaOH in solid form are introduced at a rate such that thetemperature does not exceed 60°. 217 parts of 4-nitrotoluene-2-sulfonicacid are then introduced over the course of 5 minutes into the warmsodium hydroxide solution. The temperature is subsequently raised to 74°over the course of 1 hour and 100 parts of water are added dropwise overthe course of a further hour. The pH of the reaction mixture is morethan 12. It is stirred at from 70 to 75° for a further 4 hours and then650 parts of water are added and, over the course of 25 minutes,concentrated sulfuric acid is added in an amount such that all of thedye precipitates. Thereafter, the reaction mixture is stirred withheating for a further 30 minutes.

As described in example 1, ultrafiltration and acidic washing withdilute sulfuric acid gives a low-salt dye form which, followingconcentration and neutralization with diethanolamine, gives a stableliquid formulation of the dye Direct Yellow 11.

EXAMPLES 5-49

The following table contains further dyes of which storage-stableconcentrated solutions can be prepared by ultrafiltration in accordancewith the procedure of examples 1-3. The names of the dyes refer to theColour Index, Third Edition, Volume 2 (The Society of Dyers andColourists, 1971).

Example Dye  5 Direct Yellow 27  6 Direct Yellow 127  7 Direct Yellow132  8 Direct Yellow 137  9 Direct Orange 15 10 Direct Yellow 142 11Direct Yellow 4 12 Direct Yellow 148:1 13 Direct Yellow 153 14 DirectYellow 157 15 Direct Yellow 6 16 Direct Yellow 169 17 Direct Orange 2618 Direct Red 16 19 Direct Red 23 20 Direct Red 31 21 Direct Red 238 22Direct Red 252 23 Direct Red 253 24 Direct Red 254 25 Direct Red 262 26Direct Violet 9 27 Direct Violet 51 28 Direct Violet 66 29 Direct Violet99 30 Direct Yellow 51 31 Direct Yellow 86 32 Direct Yellow 154 33Direct Orange 118:1 34 Direct Red 80 35 Direct Red 239 36 Direct Violet35 37 Direct Blue 67 38 Direct Blue 75 39 Direct Blue 78 40 Direct Blue80 41 Direct Blue 218 42 Direct Blue 267 43 Direct Blue 273 44 DirectBlue 281 45 Direct Blue 290 46 Direct Blue 301 47 Direct Blue 86 48Direct Blue 199 49 Direct Black 22 50 Direct Black 168 51 Direct Blue 86

1. A process for preparing concentrated solutions or suspensions ofanionic organic compounds, which comprises a) acidifying an aqueoussolution or suspension of an anionic organic compound containing saltsand/or impurities to a pH of 4.5 or less, if the pH is above this value,so that b) the anionic organic compound is water-insoluble andprecipitates in the form of the free acid, c) bringing the suspension toa salt content below 2% by weight, based on the total weight of theretentate, by means of ultrafiltration with a membrane having porediameters of from 0.001 to 0.02 μm, and d) optionally washing out thesalts with water having a pH below 4.5, e) optionally carrying outacid-free washing with water thereafter, and then f) concentrating, sothat the amount of anionic organic compound is 5-50% by weight, and g)if desired, bringing the anionic organic compound into solution byadding an appropriate base.
 2. A process according to claim 1, wherein adye, optical brightener or intermediate for the preparation thereof isused as anionic organic compound.
 3. A process according to claim 2,wherein dyes containing at least one sulfonic acid and/or carboxylicacid group, from the following dye classes, are used: metal-free ormetallic monoazo, disazo and polyazo dyes, pyrazolone, thioxanthone,oxazine, stilbene, formazan, anthraquinone, nitro, methine,triphenylmethane, xanthone, naphthazarine, styryl, azastyryl,naphthoperinone, quinophthalone, and phthalocyanine dyes.
 4. A processaccording to claim 3, wherein azo dyes containing at least one sulfogroup are used.
 5. A process according to claim 4, wherein a dye of theformula

is used in which KK denotes the radical of a coupling component.
 6. Aprocess according to claim 5, wherein a dye of the formula (1) is usedin which KK is a coupling component of the formula

in which Y₁ and Y₂ independently of one another are ═O, ═NH, or═N—C₁-C₄alkyl, Y₃ is ═O, ═S, ═NR or ═N—CN, where R is hydrogen orC₁-C₄alkyl, and R₁ and R₂ independently of one another are eachhydrogen, unsubstituted or substituted alkyl or unsubstituted orsubstituted phenyl.
 7. A process according to claim 6, wherein a dye ofthe formula (1) is used in which KK is a coupling component of theformula (2) in which R₁ and R₂ are hydrogen or C₁-C₄alkyl, Y₁ and Y₂ are═O or ═NH, and Y₃ is ═O, ═S, ═NH or ═N—CN.
 8. A process according toclaim 1, wherein the dye Direct Yellow 11, Direct Yellow 6 or DirectOrange 15 is used.
 9. A process according to claim 2, wherein an opticalbrightener containing sulfo and/or carboxyl groups and from one of thefollowing classes is used: bistriazinylaminostilbenes,bristriazolylstilbenes, bisstyrylbiphenyls or bisbenzofuranylbiphenyls,bisbenzoxalyl derivatives, bisbenzimidazolyl derivatives, coumarinderivatives or pyrazoline derivatives.
 10. A process according to claim9, wherein the optical brightener used is


11. A process according to claim 2, wherein an aromatic sulfonic acidwhich also carries one or more further substituents from the groupconsisting of amino, nitro, alkyl and hydroxyl is used as anionicintermediate.
 12. A process according to claim 11, wherein2-amino-5-hydroxynaphthalene-7-sulfonic acid, 4-aminotoluene-2-sulfonicacid, dehydroparathiotoluidineulfonic acid,4,4′-diaminostilbene-2,2′-disulfonic acid,4,4′-dinitrostilbene-2,2′-disulfonic acid,4,4′-diaminodiphenylamine-2-sulfonic acid or 4- nitrotoluene-2-sulfonicacid is used.
 13. A process according to claim 1, which starts from anaqueous synthesis solution or synthesis suspension containing, inaddition to the anionic organic compound, greater or lesser amounts ofstarting materials, byproducts, salts or other impurities.
 14. A processaccording to claim 13, wherein in the salts of the anionic organiccompound in the synthesis solution or synthesis suspension first of allsome or all sulfo and/or carboxyl groups are converted into the freeacid.
 15. A process according to claim 1, wherein the ultrafiltration iscarried out between room temperature and about 95° C.
 16. A processaccording to claim 1, wherein the ultrafiltration is carried out at apressure of between 2 and 10 bar.
 17. A process according to claim 1,wherein, following the ultrafiltration, the low-salt or salt-freesuspension obtained is admixed with LiOH, NH₄OH, or an organic amine.18. A process according to claim 17, wherein a C₄-C₁₂trialkylamine,C₄-C₁₂diamine, C₂-C₁₅alkanolamine or polyglycolamine is used as organicamine.
 19. A solution of anionic organic compounds, obtained by aprocess according to claim
 18. 20. A process for dyeing or opticallybrightening paper, which comprises contacting paper with an effectiveamount of a solution as described in claim 19 in which the anionicorganic compounds comprise at least one anionic dye or opticalbrightener.